April 2013

Sun and Moon
LAST QUARTER (half moon in the morning sky) falls on the 3rd. NEW MOON is on the 10h at 11:35 AM. FIRST QUARTER (half moon in the evening sky) falls on the 18th. FULL MOON occurs on the 25th at 21:57 PM.

On the 11h the young lunar crescent will be easily visible over most of the world, excluding the southern parts of South America, Australia, and New Zealand. It may be visible with the naked eye under perfect conditions in northern South Africa and after first using an optical aid to locate it from southern South Africa on the 11th, however, it will be easily visible without aid from all of South Africa on the 12th.

On the 16th at 00:23 AM the Moon will be at apogee (furthest from Earth) at a distance of about 404 864 km. The Moon will be at perigee (closest approach to Earth) at a distance of about 362 267 km on the 27h at 21:49 PM.

Planetary and Other Events – Morning and Evening
Mercury is visible in the early morning throughout the month. Venus shines as a brilliant evening star towards the end of the month. Mars is visible in the early evening twilight at the beginning of the month, but is lost in the glare of the Sun by month’s end. Jupiter is visible from dusk until around 21:20 PM at the beginning of the month, although by month’s end it will set by 19:50 PM. Saturn rises after 20:00 PM at the beginning of the month (before sunset by month’s end) and is up all night.

Uranus and Neptune are both too faint to observe with the naked eye, but may be seen using binoculars or a small telescope. Uranus is unobservable at the beginning of the month due to its proximity to the Sun. However, by month’s end it will be visible from around 05:00 AM. Neptune rises early morning, around 03:00 AM at the beginning of the month, rising progressively earlier and ealier each night, until at month’s end it rises around 01:30 AM.

Four meteor showers are active in April, the delta-Pavonids, April Lyrids, pi Puppids and the eta Aquarids. Observing prospects for the April Lyrids and pi Puppids are poor due to the full Moon on the 25th. The delta-Pavonids are active from the 11th March to the 16th April peaking on the 6th April. They are best viewed between 02:00 AM and 04:30 AM looking towards the constellation of Pavo (the Peacock). The eta Aquariids are active from the 21st April to the 12th of May, peaking on the 5th May. They are best viewed between 04:00 AM and 05:30 AM looking towards the constellation of Aquarius.

The Evening Sky Stars
The bright stars near the summer Milky Way are still prominent in the sky in April evenings, but are further to the west than in March. Stars rise about 2 hours earlier and set about 2 hours earlier than they did on the same day of the previous month — our point of view keeps changing as Earth goes around the Sun. The Milky Way gets more and more impressive as the night progresses, as the centre of our galaxy rises in the east and eventually passes overhead before dawn.

Orion can still be seen in the NW in early evening, with Taurus the Bull charging at him from low in the NW. Since Orion is upside down for us in the southern hemisphere, we see Sirius (brightest star in the sky and the eye of Orion’s Large Dog) ‘above’ Orion, with the dim stars of the Hare just above Orion’s feet. Between Orion and the stars of Leo the Lion (rising in the NE in early evening) are the dim stars of the Unicorn and the Crab, with Procyon (brightest star in Orion’s Small Dog) between them, while below Procyon are the ‘Twin’ stars Castor and Pollux low in the NNW.

In the NNE at the end of evening twilight is the upside-down question mark of Leo the Lion, representing the Lion’s head and mane. Brightest of Leo’s stars is Regulus, the ‘prince’ and one of the four ‘royal stars’, Second-brightest among Leo’s stars is Denebola (‘tail of the lion’), well to the east (right, for an observer facing north) of the ‘question mark’. According to Egyptian legend, the sun was in Leo immediately after the Creation, near Denebola. On a more scientific note, Denebola is about 36 light years away, and 12 times as bright as our sun. The dust clouds around this star suggest that planets may have formed around it, but none have been detected yet.

Blue-white Spica, brightest star in Virgo, is often shown as an ear of wheat held by a young woman, who in Babylonian mythology was queen of the stars and the lover of the god of grain. Above her in the eastern sky fly the four stars of the Crow. The head of the water snake (Hydra) is a small group of stars between Leo and Orion’s Big Dog. The rest of the snake winds its way down toward the Crow, with its brightest star, Alphard, heart of the serpent, high in the northwest. Above and to the left of the Crow, for an observer facing east, is the Cup. Alphard is an Arabic name meaning the ‘solitary one’, as there are no other bright stars near it. At about 40 times the diameter of the sun and 400 times as bright, Alphard is one of the ‘bright giants’ in our neighbourhood. But our ‘neighbourhood’ is rather large. Alphard is 11 million times as far away from us as our own sun – so it looks a lot dimmer to us!

To the south of Sirius, and nearly overhead, is Canopus, second brightest star in the sky, lighting the keel of the great ship Argo. If they were at the same distance, Canopus would appear far brighter than Sirius. Canopus is 15000 times as luminous as the sun, a rare yellow-white supergiant 313 light years away. If placed at the center of our solar system, its surface would be three quarters of the distance from the centre to the orbit of Mercury, Earth would be global toast, and so would we.

Rising in the southeast in April evenings are the stars of the Southern Cross and the Pointers (Alpha and Beta Centauri). The Alpha Centauri seen by the eye as a single bright star is easily separated by a small telescope into a brilliant pair of yellow stars, Alpha Centauri A and B. The brighter one is about half again as bright as our sun, the fainter about half as bright as the sun. Separated by about 23 times the distance from the Earth to the sun, they take 80 years to orbit each other. For many years it was believed that these were the two nearest stars to our own sun. But a third star also appears to be a member of the Alpha Centauri system, and is slightly closer to us (4.22 vs. 4.35 light years). This star, ‘Proxima Centauri’, was discovered in 1915 by Robert Innes of the Union Observatory in Johannesburg. It’s about 1/7 of the mass and diameter of the sun, and so dim that even as seen from Alpha Centauri A and B, it would be only dimly visible to the naked eye. If Proxima is in orbit around stars A and B, it probably takes about 500 000 years to go around once.

Achernar (the Little Horn) shines in the southwest in early evening, with the Magellanic Clouds nearby. These dimly glowing patches (actually nearby galaxies) were seen by some African peoples as hunting grounds for the gods, and their visibility was used to predict the weather. To the extent this worked, it probably related to the amount of dust in the air.

The Morning Sky Stars
Achernar shines brightly in the SE before dawn, with the celestial river Eridanus flowing down to the SE horizon. The Crane, the Phoenix, the Toucan, the Bird of Paradise and the Peacock are all in neighbouring parts of the southern sky, while the Sea Serpent is also in the vicinity. The stars of the Pointers and the Cross are still visible low in the SW, with the stars of the Altar, the Level and the Wolf lying between the Pointers and the Scorpion.

Almost overhead in the predawn sky at the beginning of the month is the centre of our Milky Way galaxy, with the constellations of the Archer and the Scorpion. The dark patches in the Milky Way are dust clouds that block the light of the stars behind them. It’s in the heart of dark clouds like these that new stars are constantly being born. The Khoikhoi and San said the Milky Way came into being when a strong-willed girl became angry and grabbed the roasting roots (of which her mum did not want to give her any) from the fire and threw the ashes and roots into the sky. The red and white roots now glow as red and white stars and the ashes are the Milky Way.

High in the east is Fomalhaut, brightest star in the Southern Fish, with the dimmer stars of the Sea Goat and the Water Carrier just to the north. Further to the north we get more birds. Fairly high in the north is Altair, brightest star in the Eagle and the 11th brightest star in the sky, while very low in the N to NE is the Swan, with the bright star Deneb just above our horizon. Low in the northern sky is brilliant Vega, about 25 light years from us and accompanied, as with Denebola and a number of other youngish stars, by a disk of dust which may be a leftover from the formation of a system of planets.

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